Light emitting diode structure utilizing zinc oxide nanorod arrays on one or more surfaces, and a low cost method of producing such zinc oxide nanorod arrays

CROSS REFERENCE TO RELATED APPLICATIONS

0001This application claims priority under 35 U.S.C. §119(e) of the following commonly-assigned U.S. Provisional patent applications:

0002U.S. Provisional Patent Application Ser. No. 61/257,811, entitled “A LIGHT EMITTING DIODE STRUCTURE UTILIZING ZINC OXIDE NANOROD ARRAYS ON ONE OR MORE SURFACES, AND A LOW COST METHOD OF PRODUCING SUCH ZINC OXIDE NANOROD ARRAYS,” filed on Nov. 3, 2009, by Jacob J. Richardson, Daniel B. Thompson, Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, Steven P. DenBaars, and Shuji Nakamura;

0003U.S. Provisional Application Ser. No. 61/257,812, filed on Nov. 3, 2009, by Daniel B. Thompson, Jacob J. Richardson, Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, and Steven P. DenBaars, and Shuji Nakamura, entitled “HIGH BRIGHTNESS LIGHT EMITTING DIODE COVERED BY ZINC OXIDE LAYERS ON MULTIPLE SURFACES GROWN IN LOW TEMPERATURE AQUEOUS SOLUTION;” and

0004U.S. Provisional Application Ser. No. 61/257,814, filed on Nov. 3, 2009, by Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars, Frederick F. Lange, and Jin Hyeok Kim, entitled “LIGHT EMITTING DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT EXTRACTION LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS SOLUTION;”

0005which applications are incorporated by reference herein.

0006This application is related to the following co-pending and commonly-assigned U.S. patent applications:

0007U.S. Utility patent application Ser. No. 12/761,246, filed on Apr. 15, 2010, by Jacob J. Richardson and Frederick F. Lange, entitled “LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FOR THE AQUEOUS SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLE CRYSTALS,” which application claims priority under 35 U.S.C. §119(e) to commonly-assigned U.S. Provisional Patent Application Ser. No. 61/169,633, filed on Apr. 15, 2009, by Jacob J. Richardson and Frederick F. Lange, entitled “LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FOR THE AQUEOUS SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLE CRYSTALS;”

0008U.S. Utility application Ser. No. 12/938,948, filed on Nov. 3, 2010, by Daniel B. Thompson, Jacob J. Richardson, Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, and Steven P. DenBaars, and Shuji Nakamura, entitled “HIGH BRIGHTNESS LIGHT EMITTING DIODE COVERED BY ZINC OXIDE LAYERS ON MULTIPLE SURFACES GROWN IN LOW TEMPERATURE AQUEOUS SOLUTION,” now United States Patent Publication No. 2011/0266551 published on Nov. 3, 2011, which application claims priority under 35 U.S.C. §119(e) to commonly-assigned U.S. Provisional Application Ser. No. 61/257,812, filed on Nov. 3, 2009, by Daniel B. Thompson, Jacob J. Richardson, Ingrid Koslow, Jun Seok Ha, Frederick F. Lange, and Steven P. DenBaars, and Shuji Nakamura, entitled “HIGH BRIGHTNESS LIGHT EMITTING DIODE COVERED BY ZINC OXIDE LAYERS ON MULTIPLE SURFACES GROWN IN LOW TEMPERATURE AQUEOUS SOLUTION;” and

0009U.S. Utility application Ser. No. 12/939,044, filed on Nov. 3, 2010, by Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars, Frederick F. Lange, and Jin Hyeok Kim, entitled “LIGHT EMITTING DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT EXTRACTION LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS SOLUTION,” now United States Patent Publication No. 2011-0101414, published on May 5, 2011, which application claims priority under 35 U.S.C. §119(e) to commonly-assigned U.S. Provisional Application Ser. No. 61/257,814, filed on Nov. 3, 2009, by Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars, Frederick F. Lange, and Jin Hyeok Kim, entitled “LIGHT EMITTING DIODES WITH ZINC OXIDE CURRENT SPREADING AND LIGHT EXTRACTION LAYERS DEPOSITED FROM LOW TEMPERATURE AQUEOUS SOLUTION;” which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

00101. Field of the Invention

0011This invention relates to high brightness Light Emitting Diodes (LEDs) and methods and devices that improve light extraction efficiency of LEDs without substantially increasing device cost.

00122. Description of the Related Art

0013(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)

0014There are several previous publications dealing with the use of Zinc Oxide (ZnO) nanorods for enhancing light extraction efficiency from GaN LEDs. Zhong et al. report on a metal organic chemical vapor deposition (MOCVD) grown ZnO nanorod array, on top of a ZnO:Ga transparent contact layer deposited on (0001) p-GaN [1]. Compared to a conventional Ni/Au contact, they report a ˜1.7 times increase in light emission efficiency.

0015Also using MOCVD (or MOVPE) growth, An et al. then reported on the light output from the same device, before and after growing ZnO nanorod arrays on the surface [2]. Here, the nanorods reportedly produced a ˜50% increase in output at 20 mA, and a ˜100% increase at 50 mA. Despite these improvements in light extraction, these methods are unlikely to see commercial application due to the high cost and low throughput that would be associated with using MOCVD growth for of ZnO nanorods.

0016However, Kim et al. recently reported on the growth of a ZnO nanorod array using an aqueous solution route [3]. In this case, the nanorods were able to improve light output by almost 60%. Compared with MOCVD growth, aqueous solution deposition can be a lower cost and higher throughput method [4]. This could drastically reduce the cost and make ZnO nanorod arrays a commercially feasible option for enhancing the light extraction efficiency of GaN LEDs.

0017All of the reports described above dealt solely with ZnO nanorod arrays situated on the c⁺-plane (0001) surface of the LED, either deposited directly onto the p-GaN or onto an intermediate layer. However, according to the present invention this is not the only type of LED device surface where ZnO nanorods could potentially enhance light extraction. For instance, in most laser lift off device configurations nanorods would instead need to be grown on the c⁻-plane (000-1).

0018For devices grown on bulk GaN substrates, the present invention describes that ZnO nanorod arrays could potentially improve light extraction from any of the surfaces of the device. Depending on the LED's orientation and cut, this could include the polar c⁺ and c⁻-planes, the non-polar m- and a-planes, and numerous semi-polar planes. In some cases it may also be useful to produce nanorods on top of non-GaN LED device surfaces, such as substrates, and transparent contact or current spreading layers.

SUMMARY OF THE INVENTION

0019Light extraction technology has proven to be critical for obtaining high brightness, high efficiency LEDs. To improve light extraction efficiency, surface texturing or roughening methods are often used to expand the effective escape cone for light generated within the LED. Recently, several reports have indicated that surface arrays of ZnO nanorods can help increase the light output power from GaN based LEDs. These reports have focused exclusively on using nanorod arrays only on the (0001) p-GaN surface of the LED structure. However, the (0001) is not the only surface at which light can be internally reflected. Therefore, it should be possible to enhance light extraction by using ZnO nanorod arrays on other surfaces as well.

0020The present invention introduces a new LED structure utilizing ZnO nanorods on surfaces other than the (0001) surface, and/or on multiple surfaces of the LED. The present invention also shows that such ZnO nanorod arrays can be synthesized using low cost solution processing methods. Using this type of method, nanorod arrays can be produced on various LED device surfaces, including both Ga and N faced c-plane surfaces, non-polar and semi-polar surfaces, and even non-GaN surfaces, such as substrates and transparent current spreading layers. This method of forming ZnO nanorod arrays can be applied to enhancing light output power from both lateral and vertical type LEDs.

0021To overcome the limitations in the prior art, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method of fabricating an LED with improved light extraction efficiency, comprising growing a plurality of Zinc Oxide (ZnO) nanorods on one or more light transmitting surfaces of a III-Nitride based LED, such that the nanorods are on average oriented normal the light transmitting surface on which they are grown, and wherein the nanorods are grown on at least one light transmitting surface that is different from a c⁺-plane surface of the III-Nitride based LED.

0022The method may further comprise growing a plurality of Zinc Oxide (ZnO) nanorods on one or more light transmitting surfaces of a III-Nitride based LED, such that the nanorods are on average oriented normal the light transmitting surface on which they are grown, while also growing an epitaxial ZnO layer on one or more different III-Nitride surfaces of the same III-Nitride LED.

0023The growth of the ZnO nanorods and layers may be performed in one or more steps, wherein at least one step involves growing ZnO from an aqueous solution. The aqueous solution used for ZnO growth may contain dissolved Zn(II) provided by dissolving a soluble Zn(II) salt or by dissolving ZnO. The growth from aqueous solution may be performed at a temperature of less than 100° C. Alternatively, the growth from aqueous solution may be performed at a temperature less than the boiling point of the aqueous solution, wherein that boiling point may be less than or greater than 100° C. The ZnO nanorods of layers may be formed from aqueous solution by a chemical reaction of the dissolved Zn(II) to form ZnO, wherein the chemical reaction is caused, intensified, or otherwise controlled by an increase in the temperature of the growth solution, or by a change in the pH of the growth solution. The conditions, e.g., temperature, constituent concentrations, or pH, of the aqueous growth step may be used to control the orientation, size, and/or shape of the ZnO nanorods and or ZnO layer produced.

0024The growth of the ZnO layers may include a seed layer deposition step where a nano-crystalline seed layer is formed on one or more surfaces of the III-Nitride LED. A seed layer may be deposited from aqueous solution as described above. Alternatively, a seed layer may be deposited by first depositing a Zn(II) precursor dissolved in a solvent on one or more surfaces to form precursor films, followed by annealing the precursor film to convert the precursor film to a nano-crystalline ZnO film. Alternatively, the nanocrystalline seed may be deposited using a vapor phase method, e.g., sputtering, evaporation, or chemical vapor deposition. The growth of ZnO nanorods or layers may comprise converting the one or more nano-crystalline seed layers by growing further ZnO on the seed layer from an aqueous solution, as described above. The conditions used for the deposition of the Zn(II) precursor film, or for the conversion of the Zn(II) precursor film into a nano-crystalline ZnO layer, may be used to control the orientation, size, and/or shape or the final ZnO nanorods or ZnO layers.

0025The orientation, size, and shape of the ZnO nanorods and/or ZnO layers grown on light transmitting surfaces of the III-Nitride LED may be controlled to enhance the light extraction from those surfaces of the LED.

0026The light transmitting surfaces may include an N faced c-plane surface, a non-polar plane, and/or a semi-polar plane III-Nitride surface, and/or a non-III-nitride surface of the III-nitride based LED.

0027The ZnO nanorods may be grown on the III-nitride LED that is fully fabricated, and the growing step may be a final step prior to encapsulation of the LED.

0028The present invention further discloses an LED, comprising a plurality of ZnO on one or more light transmitting surfaces of a III-Nitride based LED, such that the nanorods are on average oriented normal the light transmitting surface on which they are grown, and wherein the nanorods are grown on at least one light transmitting surface that is different from a c⁺-plane surface of the III-Nitride based LED.

0029The nanorods may be on a plurality of the surfaces of the III-Nitride LED. The device may comprise a number of the ZnO nanorods, each with one or more dimensions, wherein the number and dimensions enhance extraction of the light from the LED as compared to an LED without the ZnO nanorods. The LED may be a polar, non-polar or semi-polar LED.

BRIEF DESCRIPTION OF THE DRAWINGS

0030Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

0031<figref idref="DRAWINGS">FIG. 1</figref> and <figref idref="DRAWINGS">FIG. 2</figref> show cross-sectional schematics of examples of LED structures utilizing ZnO nanorod arrays on multiple surfaces, wherein <figref idref="DRAWINGS">FIG. 1</figref> shows a lateral type device, and <figref idref="DRAWINGS">FIG. 2</figref> shows a vertical type device.

0032<figref idref="DRAWINGS">FIG. 3(</figref><i>a</i>) is a flowchart illustrating a first embodiment of a method of synthesizing ZnO on an LED, and <figref idref="DRAWINGS">FIG. 3(</figref><i>b</i>) is a flowchart illustrating a second embodiment of a method of synthesizing ZnO on an LED.

0033<figref idref="DRAWINGS">FIGS. 4(</figref><i>a</i>) and <b>4</b>(<i>b</i>) show nanorods grown with a low temperature aqueous solution method on the non-polar GaN surface of a bulk GaN substrate LED.

0034<figref idref="DRAWINGS">FIG. 5(</figref><i>a</i>) is a cross-sectional schematic of an LED comprising epitaxial and non-epitaxial structures for light extraction, and <figref idref="DRAWINGS">FIG. 5(</figref><i>b</i>) is a scanning electron micrograph (SEM) image of the LED in <figref idref="DRAWINGS">FIG. 5(</figref><i>a</i>).

DETAILED DESCRIPTION OF THE INVENTION

0035In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

0036Overview

0037The purpose of the current invention is to enhance the power output of GaN LED devices. Due to growing energy concerns, efficient LED lighting technology is of current and expanding importance. However, the cost of LED lighting remains high and is a major impediment to the greater implementation of the technology. This invention allows the power output and/or light extraction efficiency of LEDs to be increased without substantially increasing the cost of the device.

0038Technical Description

0039Nomenclature

0040III-nitrides may be referred to as group III-nitrides, nitrides, or by (Al,Ga,In)N, AlInGaN, or Al_(1-x-y)In_yGa_xN where 0&lt;x&lt;1 and 0&lt;y&lt;1, for example.

0041These terms are intended to be broadly construed to include respective nitrides of the single species, Al, Ga, and In, as well as binary, ternary and quaternary compositions of such Group III metal species. Accordingly, the terms comprehend the compounds AN, GaN, and InN, as well as the ternary compounds AlGaN, GaInN, and AlInN, and the quaternary compound AlGaInN, as species included in such nomenclature. When two or more of the (Ga, Al, In) component species are present, all possible compositions, including stoichiometric proportions as well as “off-stoichiometric” proportions (with respect to the relative mole fractions present of each of the (Ga, Al, In) component species that are present in the composition), can be employed within the broad scope of the invention. Accordingly, it will be appreciated that the discussion of the invention hereinafter in primary reference to GaN materials is applicable to the formation of various other (Al, Ga, In)N material species. Further, (Al,Ga,In)N materials within the scope of the invention may further include minor quantities of dopants and/or other impurity or inclusional materials. Boron may also be included in the III-nitride alloy.

0042Similarly, the terms zinc oxide or ZnO are intended to be broadly construed to include any material where the component species Zn and O make up the majority of the compound, and the material retains the hexagonal Wurtzite crystal structure of ZnO. This is inclusive of aluminum doped zinc oxide (AZO), gallium doped zinc oxide (GZO), and indium doped zinc oxide (IZO). This also includes materials with minor quantities of other dopants and/or other impurity or inclusional materials, as well as materials that are off-stoichiometric due to the presence of vacancy and interstitial type material defects.

0043Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.

0044One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN or III-nitride optoelectronic devices is to grow the devices on nonpolar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN or III-nitride are the {11-20} family, known collectively as a-planes, and the {1-100} family, known collectively as m-planes.

0045Another approach to reducing or possibly eliminating the polarization effects in GaN optoelectronic devices is to grow the devices on semi-polar planes of the crystal. The term “semi-polar planes” can be used to refer to a wide variety of planes that possess both two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Thus, semipolar planes are defined as crystal planes with nonzero h or k or i index and a nonzero/index in the (hkil) Miller-Bravais indexing convention. Some commonly observed examples of semi-polar planes in c-plane GaN heteroepitaxy include the (11-22), (10-11), and (10-13) planes, which are found in the facets of pits. Other examples of semi-polar planes in the wurtzite crystal structure include, but are not limited to, (10-12), (20-21), and (10-14). The nitride crystal's polarization vector lies neither within such planes or normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the (10-11) and (10-13) planes are at 62.98° and 32.06° to the c-plane, respectively.

0046The Gallium or Ga face of GaN is the c⁺ or (0001) plane, and the Nitrogen or N-face of GaN or a III-nitride layer is the c⁻ or (000-1) plane.

0047The term nanorod is construed as a structure with a maximum diameter less than 1000 nm in the one (lateral) direction and a similar or greater dimension in the orthogonal (longitudinal) direction giving the structure an aspect ratio greater or equal to 1, wherein one longitudinal end of the structure is terminated on a substrate.

0048LED Structure and Fabrication

0049The present invention describes a III-Nitride LED structure with ZnO nanorods deposited on a non-c⁺-plane III-Nitride surface, and/or on multiple surfaces of the device, for enhanced light extraction and/or heat dissipation. In addition, the present invention describes a low cost solution based process for synthesizing the nanorod arrays in such a structure.

0050<figref idref="DRAWINGS">FIG. 1</figref> and <figref idref="DRAWINGS">FIG. 2</figref> show schematic diagrams of bulk GaN substrate <b>100</b>, <b>200</b> type LED devices <b>102</b>, <b>202</b> utilizing or comprising ZnO nanorod <b>104</b>, <b>204</b> arrays on multiple surfaces <b>106</b><i>a</i>, <b>106</b><i>b</i>, <b>206</b><i>a</i>, <b>206</b><i>b</i>, <b>206</b><i>c</i>. <figref idref="DRAWINGS">FIG. 1</figref> shows a lateral type LED <b>102</b> and <figref idref="DRAWINGS">FIG. 2</figref> shows a vertical type LED <b>202</b> with reflective p-type contact layer <b>208</b>.

0051Multiple variations of these types of structure can be made and multiple processing methods could be used to realize these types of structures. Here, the present invention describes in detail a method where a working LED device (including n-type GaN layer <b>108</b>, <b>210</b>, p-type GaN layer <b>110</b>, <b>212</b>, active region <b>112</b>, <b>214</b>, n-type or n-contact(s) <b>114</b>, <b>216</b>, p-type or p-contact(s) <b>116</b>, <b>208</b>, and p-pad layers <b>118</b>) on a bulk GaN substrate <b>100</b>, <b>200</b> is first fully fabricated, and then the ZnO nanorod <b>104</b>, <b>204</b> structures are grown as a final step just before encapsulation.

0052The present invention fabricated the lateral type LED <b>102</b> of <figref idref="DRAWINGS">FIG. 1</figref> with a typical processing method. First, a region of the LED wafer (comprising layers <b>108</b>, <b>110</b>, <b>112</b>) is mesa-etched to form a mesa <b>120</b>, removing some of the p-GaN (the p-type GaN layer) <b>110</b> and active region layers <b>112</b> from the surrounding area using a Cl₂reactive ion etch, and exposing the n-type GaN layer <b>108</b>. Following that, a p-contact layer (p-type contact) <b>116</b> such as Ni/Au, ITO, or ZnO is deposited on the p-type GaN layer <b>110</b>, and metal contacts <b>114</b> of Ti/Al/Ni/Au are deposited on the exposed n-type GaN <b>108</b>, to form the n-contact and pad layer. A p-pad layer <b>118</b> is also deposited on the p-contact layer <b>116</b>.

0053For the vertical type LED (<figref idref="DRAWINGS">FIG. 2</figref>) <b>202</b>, no mesa forming step is necessary, and a highly reflective p-contact layer <b>208</b> such as Ag, Al or Rh is deposited directly on the p-GaN <b>212</b>. These metals can be combined in stacks with higher work function materials to decrease the contact resistance. Also shown in <figref idref="DRAWINGS">FIG. 2</figref> is metal support <b>218</b>.

0054The individual devices are then cut from the wafer so that ZnO nanorod <b>104</b>, <b>204</b> arrays can be fabricated on multiple exposed surfaces <b>106</b><i>a</i>-<i>b</i>, <b>206</b><i>a</i>-<i>c </i>of the final device.

0055Specifically, a plurality ZnO nanorods <b>104</b>, <b>204</b> are (e.g., formed non-epitaxially) on one or more light transmitting surfaces <b>106</b><i>a</i>-<i>b</i>, <b>206</b><i>a</i>-<i>c </i>of a III-Nitride based LED, wherein (1) the light transmitting surfaces <b>106</b><i>a</i>, <b>106</b><i>b</i>, <b>206</b><i>a</i>-<i>c </i>with the ZnO nanorods <b>104</b>, <b>204</b> include one or more surfaces that are different from a c⁺-plane surface of the III-Nitride LED <b>102</b> (e.g., different from a c⁺ plane of the p-type layer of the III-Nitride based LED), (2) the nanorods <b>104</b>, <b>204</b> are oriented such that on average the longitudinal axis <b>120</b><i>a</i>, <b>120</b><i>b</i>, <b>220</b><i>a</i>, <b>220</b><i>b</i>, <b>220</b><i>c </i>of a nanorod <b>104</b>, <b>204</b> is parallel to the [0001] (c-direction) of the ZnO nanorod's wurtzite crystal structure and perpendicular to the light transmitting surface <b>106</b><i>a</i>, <b>106</b><i>b</i>, <b>206</b><i>a</i>, <b>206</b><i>b</i>, <b>206</b><i>c </i>upon which the nanorods <b>104</b>, <b>204</b> are deposited; and (3) a number of the ZnO nanorods <b>104</b>, <b>204</b>, each with one or more dimensions (e.g., length <b>122</b>, <b>222</b> and diameter <b>124</b>, <b>224</b>) such that the number and/or density and/or dimensions enhance extraction of the light from the LED as compared to an LED without the ZnO nanorods <b>104</b>, <b>204</b>. The enhancement of extraction may be by scattering, reduced total internal reflection, diffraction, or photonic crystal effects, for example. The dimensions and/or spacing of the nanorods <b>104</b> may be sufficiently close to a wavelength of the light emitted by the LED, for example, so that the light may be scattered, diffracted, reflected, or otherwise interact electromagnetically with the nanorods.

0056In some embodiments, the ZnO nanorods <b>104</b> may be on top of a ZnO nano-crystalline seed layer, wherein the seed layer is on the light transmitting surface <b>106</b><i>a </i>so that the seed layer is between the light transmitting surface and the ZnO nanorods <b>104</b>.

0057In the embodiments shown in <figref idref="DRAWINGS">FIGS. 1 and 2</figref>, the light transmitting surfaces include at least one roughened surface <b>106</b><i>b</i>. The ZnO nanorods <b>104</b>, <b>204</b> deposited on roughened surfaces <b>106</b><i>a </i>may be oriented in a plurality of directions and enhance light extraction. However, in other embodiments, the surface is not roughened.

0058For example, the non-roughened light transmitting surface <b>106</b><i>b </i>may include an N faced c-plane surface of the III-nitride based LED.

0059In other embodiments, the light transmitting surfaces <b>106</b><i>a</i>-<i>c </i>include one or more non-polar planes and/or one or more semi-polar planes, and/or non-GaN or non-III-nitride surface(s), of the III-nitride based LED.

0060The LED <b>102</b> may be a polar c-plane LED or not a c-plane LED (e.g., a non-polar or semi-polar LED). C-plane LEDs are grown in the c-direction of the III-nitride material so that the final or top surface of the LED is a polar c-plane, whereas non-polar or semi-polar LEDs are grown in the non-polar or semi-polar direction of the III-nitride material, so that the final or top surface of the LED is a non-polar or semi-polar plane and the LED experiences reduced or eliminated polarization effects.

0061ZnO Nanorod Synthesis

0062Zinc oxide shares the Wurtzite crystal structure and is well lattice matched with GaN, a fact that will encourage the epitaxial growth of ZnO on GaN. However, to achieve the greatest light extraction, ZnO nanorods should always be oriented normal to the LED surface. Since ZnO nanorods always form with the long axis oriented in the c-direction, this means that surface normal nanorods can only be grown epitaxially on the basal planes (e.g., c-planes) of GaN, where the c-direction of the ZnO is aligned with the c-direction of the GaN.

0063Therefore, to form surface normally oriented ZnO nanorods on the other surfaces of GaN LEDs, epitaxial growth cannot be used. However, even on the basal planes of GaN, epitaxial growth may not be preferred. This is due to the fact that a wide range of heteroepitaxial growth conditions will lead to films rather than nanorods on the basal planes of GaN [5].

0064For these reasons, a method capable of growing ZnO nanorod arrays on multiple GaN surfaces should be able to inhibit epitaxial growth. If epitaxy is prevented, most of the ZnO rods that result will tend to have their longitudinal axis oriented normal to the LED surface. This occurs for several reasons. First, ZnO will tend to nucleate with a basal plane orientation to minimize surface energy. These oriented nuclei then grow into oriented rods. Secondly, oriented nanorods will tend to dominate due to the fact that non-oriented rods are self-terminating. Because ZnO grows preferentially in the c-direction, non-oriented rods will quickly run into adjacent rods, ending their growth. At the same time, oriented rods can grow unimpeded.

0065Epitaxial ZnO growth can be prevented by controlling the conditions under which nucleation occurs, or by depositing an interfacial layer to disrupt the lattice alignment between GaN and ZnO. In the procedure detailed below, epitaxy is minimized by only controlling the conditions for nucleation and growth. However, the deposition of an interfacial layer could be added without significantly modifying the rest of the procedure.

0066Block <b>300</b> of <figref idref="DRAWINGS">FIG. 3(</figref><i>a</i>) illustrates a method of fabricating an LED with improved light extraction efficiency, comprising growing a plurality of ZnO nanorods on one or more light transmitting surfaces of a III-Nitride based LED, wherein the ZnO nanorods are grown on at least one light transmitting surface that is different from a c⁺-plane surface (e.g., growing the nanorods on non-c⁺ plane surfaces) of the III-Nitride based LED, or the LED is not a c-plane LED.

0067The growing of the ZnO nanorods may be from an aqueous solution containing dissolved Zn(II), by means of a chemical reaction involving the dissolved Zn(II), wherein the ZnO nanorods are formed on a surface of the III-Nitride LED in contact with the aqueous solution.

0068The orientation, size, and shape of the ZnO nanorods and/or ZnO layers grown on light transmitting surfaces of the III-Nitride LED may be controlled to enhance the light extraction from those surfaces of the LED.

0069The growing may be under conditions to inhibit epitaxial growth on the LED, by controlling the conditions under which nucleation occurs, or by depositing the interfacial layer between the LED and the ZnO to disrupt lattice alignment between the LED material and the ZnO. The growing may be from an aqueous solution, and the growing may be at a temperature below the boiling point of the aqueous solution such that the aqueous solution is a liquid aqueous solution. The growing may be from an aqueous solution at a temperature below 100° C., for example.

0070The ZnO nanorods may be grown on the III-nitride LED that is fully fabricated, and the growing step may be a final step prior to encapsulation of the LED. Block <b>302</b> represents optionally depositing one or more epitaxial ZnO layers on one or more surfaces of the III-Nitride LED that are different from the light transmitting surfaces where the ZnO nanorods are grown.

0071Block <b>304</b> represents optionally roughening the epitaxial ZnO layers to enhance light extraction from the LED.

0072Block <b>306</b> represents the end result of the method, a device. The device may comprise a plurality of Zinc Oxide (ZnO) nanorods on one or more light transmitting surfaces of a III-Nitride based LED, wherein the light transmitting surfaces with the ZnO nanorods include at least one surface that is different from a c⁺-plane surface of the III-Nitride based LED. An epitaxial ZnO layer may optionally be on one or more surfaces of the III-Nitride LED that are different from the light transmitting surfaces where the ZnO nanorods are grown. However, as noted above, the step in Block <b>302</b> may be omitted, or the steps of Blocks <b>302</b> and <b>304</b> may be omitted, or Blocks <b>302</b> and <b>304</b> may be performed prior to Block <b>300</b>, for example.

0073In one example of the above method, ZnO nanorod arrays can be formed or grown on any arbitrary surface of an (e.g., GaN) LED using a two-step process. An example of the growing of Block <b>300</b> using the two step method is illustrated in <figref idref="DRAWINGS">FIG. 3(</figref><i>b</i>) and described below.

0074Seed Layer Deposition

0075In the first step, represented by Block <b>308</b>, a nano-crystalline seed layer is deposited or grown on the surfaces.

0076To accomplish this several methods could be used, including low temperature aqueous solution deposition, a solution based precursor deposition and conversion, or a vapor phase deposition technique. Whatever the technique used, conditions should be optimized to reduce the nucleation of epitaxial ZnO, especially on non-c-plane GaN surfaces. How this is achieved will depend on the particular method used, but in general it is important to avoid high temperatures during processing. High temperatures allow diffusion, which in can turn allow non-epitaxial ZnO to recrystallize into epitaxial ZnO. Conditions affecting the ZnO grain size in the seed layer are also of particular importance, as the seed crystal size will effectively establish the minimum ZnO rod diameter. Thus, the present invention may grow ZnO at temperatures sufficiently low to prevent diffusion and/or conditions that produce seed crystal grain sizes that allow nanorod growth.

0077<figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>) and <figref idref="DRAWINGS">FIG. 4(</figref><i>b</i>) are photographs illustrating a top view of ZnO nanorods (viewed from the end, showing the end of the nanorods) fabricated using the method of the present invention. The light spots in <figref idref="DRAWINGS">FIGS. 4</figref><i>a </i>and <b>4</b><i>b </i>illustrate the ends or tips of the ZnO rods as viewed from the top.

0078The experimental results shown in <figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>)-(<i>b</i>) were obtained using a chemical solution deposition of a precursor film followed by a conversion technique to create the nano-crystalline ZnO seed layer. For example, the nano-crystalline seed layer may be deposited by depositing a solution comprising a Zn(II) precursor dissolved in a solvent on each of the surfaces, to form a precursor film; and then heating or annealing the film to pyrolize the Zn(II) precursor and crystallize a nano-crystalline ZnO film that is the nano-crystalline seed. To create the chemical precursor solution, zinc(II) acetate dehydrate was dissolved in ethanol at a concentration of 0.5 mol/L along with 0.5 mol/L diethanolamine. The addition of diethanolamine serves to increase the solubility of the zinc(II) acetate in solution, as well modify the viscosity and drying behavior of the solution. Other solution formulations could be substituted. The precursor solution was then deposited on the desired surfaces by spin coating. The same solution could also be deposited by other methods, such as dip coating or spray coating. The resulting precursor film was then converted to nanocrystalline ZnO using a rapid thermal annealing treatment to 600° C. in an N₂/O₂atmosphere. This treatment both pyrolized and crystallized the film, but the pyrolysis and crystallization stages could also be performed separately with a lower temperature treatment followed by a higher temperature treatment.

0079Alternatively, the nanocrystalline seed may be deposited using a vapor phase method, e.g., sputtering, evaporation, or chemical vapor deposition.

0080Aqueous Conversion of ZnO Seed Layers to a Nanorod Arrays

0081After the seed layer deposition, a second step (Block <b>310</b>) grows the nanocrystalline ZnO seeds into an array of nanorods. This is accomplished by converting the nano-crystalline seed layer to the ZnO nanorods by growing in an aqueous solution, e.g., the step may comprise growing the plurality of ZnO nanorods on the seed layer, by growing the ZnO nanorods from an aqueous solution.

0082Growth of ZnO nanorods and films from aqueous solution is well known and many different specific solution conditions can be used for this purpose [10-11]. The specific solution conditions used will, along with the properties of the seed layer, determine the length, width, aspect ratio, orientation, and morphology of the ZnO nanorods formed. Thus, a number of the ZnO nanorods may be grown with a desired number and dimensions, wherein the number and dimensions enhance extraction of the light from the LED as compared to an LED without the ZnO nanorods.

0083For ZnO rods to form from a stable solution, growth may be initiated by some change in the solution conditions. This, for example, can be achieved with a change in the pressure, pH, temperature, or the concentrations of one or more of the solution components [8]. For the results shown in <figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>)-(<i>b</i>), the aqueous growth solution consisted of only zinc(II) nitrate hexahydrate, dissolved ammonia, and water. ZnO was formed from the solution after the solution is heated. To ensure that the reaction has reached completion, the solution was then maintained at 90° C. for a period of 18-24 hours. However, it is likely that the majority of the growth occurs fairly quickly and that the results would be relatively unaffected by significantly shortening the hold time.

0084The conditions used for the deposition of the Zn(II) precursor film (Block <b>308</b>), or for the conversion (Block <b>310</b>) of the Zn(II) precursor film into a nano-crystalline ZnO layer, may be used to control the orientation, size, and/or shape or the final ZnO nanorods or ZnO layers.

0085ZnO Nanorod and Epitaxial Layers on Different Surfaces

0086<figref idref="DRAWINGS">FIG. 5(</figref><i>a</i>) is a schematic of an III-Nitride LED <b>500</b> comprising an epitaxial ZnO layer <b>502</b> on a first surface <b>504</b> of the LED <b>500</b>, and ZnO nanorods or non-epitaxial ZnO <b>506</b> on different surfaces <b>514</b> of the LED <b>500</b>. In this embodiment, the epitaxial ZnO layer <b>502</b> is grown on the nitrogen terminated (000-1) c-plane surface <b>504</b> of a GaN bulk substrate <b>506</b>, opposite of the LED device layers <b>508</b> (n-type layer, active layer, and p-type layer), which are grown on the gallium terminated (0001) c-plane surface <b>510</b> of the of the GaN bulk substrate <b>506</b>. The ZnO nanorods <b>512</b> are grown on the sidewall surfaces <b>514</b> of the bulk GaN substrate <b>506</b>. The sidewall surfaces <b>514</b> comprise of either m-plane (10-10) or a-plane (11-20) surfaces (i.e, non-polar planes). The surfaces <b>504</b> and <b>514</b> of the bulk GaN substrate <b>506</b> with epitaxial ZnO <b>502</b> or ZnO nanorods <b>512</b> may be roughened prior to ZnO growth, so that the light transmitting surfaces include at least one roughened surface <b>504</b>, <b>514</b> so that the ZnO nanorods <b>512</b> are oriented in a plurality of directions and enhance light extraction.

0087<figref idref="DRAWINGS">FIG. 5(</figref><i>b</i>) is an SEM image of the LED of <figref idref="DRAWINGS">FIG. 5(</figref><i>a</i>), showing a top surface <b>516</b> of the epitaxial ZnO layer <b>502</b> that is roughened <b>518</b>, and the non-epitaxial nanorod structures of ZnO <b>512</b> on the sidewalls <b>514</b> of the GaN bulk substrate <b>506</b>. The LED comprises multiple surfaces (epitaxial ZnO <b>502</b> and non-epitaxial nanorods <b>512</b>) adapted for light extraction.

0088The present invention is able to obtain simultaneous nucleation of epitaxial <b>502</b> and ZnO nanorods <b>506</b> on different GaN surfaces.

0089Thus, <figref idref="DRAWINGS">FIGS. 5(</figref><i>a</i>)-(<i>b</i>) illustrate an embodiment where one or more epitaxial ZnO layers <b>502</b> are deposited on one or more surfaces <b>504</b> of the III-Nitride LED <b>500</b> that are different from the light transmitting surfaces <b>514</b> where the ZnO nanorods <b>506</b> are grown. <figref idref="DRAWINGS">FIGS. 5(</figref><i>a</i>)-(<i>b</i>) further illustrate an embodiment wherein the epitaxial ZnO layers <b>502</b> are roughened <b>518</b> to enhance light extraction from the LED <b>500</b>.

ADVANTAGES AND IMPROVEMENTS

0090The present invention communicated here presents several important advantages over the current state of the art. From the prior art, it is clear that ZnO nanorod arrays applied to c-plane surfaces can impart improved light extraction characteristics to III-Nitride LED devices. By applying ZnO nanorod arrays to previously unused or uncoated surfaces, and or multiple LED surfaces, further enhancement of light extraction is anticipated. This has particular relevance to devices fabricated on bulk GaN because of the numerous types of non-c-plane surfaces that can be used and the larger relative areas of these surfaces as compared to conventional LEDs. The major advantage of the solution based method for ZnO nanorod arrays presented here is the low cost. Compared to high temperature chemical and physical vapor deposition methods, such as VLS (vapor-liquid-solid) or MOCVD, this solution route provides potential cost advantages in equipment, energy, chemical precursors, and throughput. The method presented here is suitable for deposition on to multiple types of LED device surfaces, and the relatively mild conditions used allow for versatility in the process flow schemes that are possible.

REFERENCES

0091The following references are incorporated by reference herein. <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0092">[1] J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie and H. Shen: Appl. Phys. Lett. 90 203515 (2007).</li><li id="ul0001-0002" num="0093">[2] Sung Jin An, Jee Hae Chae, Gyu-Chul Yi, and Gil H. Park: Appl. Phys. Lett. 92 121108 (2008).</li><li id="ul0001-0003" num="0094">[3] Kyoung-Kook Kim, Sam-dong Lee, Hyunsoo Kim, Jae-Chul Park, Sung-Nam Lee, Youngsoo Park, Seong-Ju Park, and Sang-Woo Kim: Appl. Phys. Lett. 94 071118 (2009).</li><li id="ul0001-0004" num="0095">[4] Frederick F. Lange: Science vol 273 issue 5277 pg. 903 (1996).</li><li id="ul0001-0005" num="0096">[5] Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars, and Frederick F. Lange, entitled “Light Emitting Diodes with ZnO Current Spreading Layers Deposited from a Low Temperature Aqueous Solution,” Applied Physics Express 2 (2009) 042101, published online on Mar. 19, 2009.</li><li id="ul0001-0006" num="0097">[6] Presentation Slides given by Jacob Richardson, entitled “Low Temperature Aqueous Deposition of ZnO on GaN LEDs,” at the 2009 Annual Review for the Solid State Lighting and Energy Center (SSLEC), University of California, Santa Barbara (Nov. 5, 2009).</li><li id="ul0001-0007" num="0098">[7] J. H. Kim, E. M. Kim, D. Andeen, D. Thompson, S. P. DenBaars, and F. F. Lange: Adv. Funct. Mater. 17 (2007) 463.</li><li id="ul0001-0008" num="0099">[8] U.S. Utility patent application Ser. No. 12/761,246, filed on Apr. 15, 2010, by Jacob J. Richardson and Frederick F. Lange, entitled “LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FOR THE AQUEOUS SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLE CRYSTALS.”</li><li id="ul0001-0009" num="0100">[9] U.S. Pat. No. 7,265,037B2, entitled “Nanowire array and nanowire solar cells and methods for forming the same,” by Yang et. al., issued Sep. 4, 2007.</li><li id="ul0001-0010" num="0101">[10] U.S. Pat. No. 7,767,140 B2, entitled “Method for manufacturing zinc oxide nanowires and device having the same,” by Min et. al., issued Aug. 3, 2010.</li></ul>

CONCLUSION

0102This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.